1. Field of the Invention
The present invention is generally related to reactivity control of nuclear reactors and in particular to active control of reactivity with a variable distributed neutron poison configuration.
2. General Background
Reactivity in nuclear reactors is regulated by adding or removing materials that either absorb (poison), moderate (slow), or reflect (modify leakage of) neutrons. Reactivity can also be controlled by adding or removing fissionable material. Reactivity can be controlled indirectly through a change in the temperature of the core materials. Temperature affects neutron absorption characteristics and thermal expansion changes the physical relationship of the core constituents which in turn changes the neutron leakage from the reactor core. The intentional change in reactivity, such as by adding poison material, is referred to as active control. A natural change in reactivity, such as may occur from an increase in reactor temperature, is referred to as passive control. A common method of active control in light water reactors is by the use of control rods or blades. The control rods contain a neutron poison such as boron carbide, silver indium-cadmium alloy, or hafnium. The rods are progressively inserted into the core from one end, absorbing neutrons and reducing reactivity in the vicinity of the rods. The size and number of rods and the amount of insertion is varied to achieve the desired level of reactivity or reactor control. Another method of active control, used mainly in compact reactors, is the rotation of control drums. Control drums are positioned in a wide band of neutron reflecting material, such as beryllium, that surrounds the core and serves to reduce leakage of neutrons from the core. Control drums are formed from a cylinder having reflector material along the full length of approximately two thirds of the cylinder and a poison material, such as boron carbide, along the length of the remainder of the cylinder. When the drum is positioned with the poison toward the center of the core, neutrons are absorbed, reducing the core reactivity. When the drum is rotated to position the reflector material toward the core, fewer neutrons are absorbed and more are reflected back into the core. This increases core reactivity, particularly around the periphery of the core. Another means of active control includes the addition of a soluble poison such as boric acid to the coolant. The concentration of boric acid is adjusted to control core reactivity to the desired level. Passive reactivity control is also utilized in most reactors to limit an overheating event. Such reactors are known to have a negative reactivity temperature coefficient or doppler coefficient. When a reactor inadvertently generates more power than can be removed, materials get hotter, expand, and in turn, allow more neutrons to leak from the core, resulting in a reduction of power that precludes continued overheating. Also, as the temperature increases the resonance absorption characteristics of the materials broaden and more neutrons are absorbed. Known reactivity control methods include the following.
U.S. Pat. No. 2,987,455 to Huston et al. discloses placing neutron absorbing material within the reactor core and triggering an increase in neutron absorbing material surface area exposed to the neutron flux. An expandible bladder coated with a neutron poison is maintained in a coiled configuration and then expanded to increase the exposed surface area of the poison on the bladder in the event of a reactivity runaway.
U.S. Pat. No. 3,309,284 to Bennett discloses the use of compressional waves of a powdered, gaseous, or liquid neutron poison as a means for controlling nuclear reactivity.
U.S. Pat. No. 3,629,061 to Noyes et al. discloses the substitution of moderator material for structural steel instead of fuel.
U.S. Pat. No. 3,799,839 to Fischer et al. discloses an arrangement of burnable poison in a nuclear reactor core wherein plutonium fuel is used in conjunction with the burnable poison.
U.S. Pat. No. 3,085,962, No. 3,366,546, No. 3,745,069, No. 4,576,787, and No. 4,645,643 disclose a variety of fuel elements and poison control assemblies.
A number of the reactivity controls known and described above are designed to be automatically activated in response to excess reactivity. This does not address the problem of achieving and maintaining uniform power distribution while adjusting core reactivity and reactor power. Ideally, power should be uniform throughout the core to help provide for efficient fuel utilization and maximum power output per unit of core volume. Typically, power peaks at the core center and falls off in both the radial and axial directions. Core power flattening and maintaining uniform power throughout the core life require that the neutron poison burn up at a rate that is compatible with that of the nuclear fuel. Due to inherently different burnup rates between poison and fuel materials, this is not easily achieved.